The present invention relates to telecommunications and more particularly to a method and means for enhancing a non-coded video image based on one bit/pel data through the selective introduction of gray-scale pels prior to presentation on a gray scale display device.
Because of rising travel costs, there is an increasing interest in the use of teleconferencing systems for the conduct of business meetings. In a typical teleconferencing system, people in different cities or even different countries meet in special teleconferencing rooms. Each room normally includes a room camera for capturing a wide angle view of the people, a document camera which can be focussed on letters, drawings or other documents, a room monitor for permitting people in one room to see those in the other, and a document monitor for viewing documents being presented in the other room. Communications between the two rooms are established over conventional teleprocessing links, such as leased or switched telephone lines or satellite communication channels.
To reduce communications costs, freeze-frame teleconferencing systems are employed. In such systems, the video image captured by a room camera is updated only periodically, perhaps on the order of once every 30 seconds. People at the receiver will see the same "frozen" room image for 30 seconds between updates. Audio signals are transmitted on a "real time" basis so that there is no significant delay in voice communications. Document images are updated only when the person presenting a document pushes a "send" button in the teleconferencing room.
After a "send" button is pushed, the image of the presented document will not appear immediately on a display or monitor in the receiving teleconferencing room. A finite period of time is required to capture and process image data at the originating teleconferencing room, to transmit the processed data over teleprocessing links and to process data at the receiving teleconferencing room in order to reconstruct the image of the presented document. The length of the delay can be critical in a teleconferencing system. Delays exceeding a few seconds will produce unnatural pauses in the smooth flow of a business meeting.
The length of the delay is generally proportional to the amount of data which must be transmitted in order to construct an acceptable video image and is inversely proportional to the bandwidth of the teleprocessing link over which the data must be transmitted. While the amount of delay can be reduced by using a higher bandwidth channel, the drawback to this approach is that communications cost are a function of required bandwidth. Therefore, it is desirable to use as little bandwidth as possible.
Attempts have been made to minimize delay time and to maintain low communication costs by compressing the amount of data which must be transmitted over a low bandwidth channel in order to reconstruct a video image of a presented document. For example, documents which are normally bilevel (i.e., black printing on white paper) can be digitized by assigning a binary value to each sampled picture element (pel). Thus, each pel would represent either black or white. When formatted in a two-dimensional array, the data would represent the captured image. For typical business documents, encoding this data using a two-dimensional run length encoding technique will significantly reduce the amount of data required for transmission.
However, bi-level sampling may distort an image in several ways. One very noticeable distortion is the creation of a stair-step appearance along non-vertical and non-horizontal lines. As the sampling resolution is decreased, so as to decrease the captured image data and improve the transmission delay, the steps or jagged edges become larger and increasingly displeasing to the viewer. For typical business documents, however, the image remains readable long after the decreased sampling resolution has produced very displeasing visual distortions. Therefore, an effective system approach is to sample, compress, and transmit low resolution bi-level data to attain high speed and low transmission costs, and to then operate on the received data after transmission to enhance the visual appearance of the image.
There are known techniques for reducing the visual impact of jagged or stepped edges.
One technique for smoothing stepped edges has been to pass a raster control signal for the receiving monitor through a low pass filter. The low pass filter removes the high frequency components from the signal, thus rounding out the corners and decreasing its rise and fall times. Unfortunately, the absence of high frequencies will cause the entire character (and not just stepped edges) to be blurred as if the receiving monitor were slightly out of focus.
Another technique for smoothing stepped edges uses delay apparatus for storing digitally generated raster control signals. Stored raster control signals are combined with more recent raster control signals in an analog fashion in order to generate a composite or interpolated raster control signal. The interpolated raster control signal is resolved in horizontal time increments of finer resolution than could be obtained from the original raster control signal. Raster control signals of several horizontal scan lines are also compared in order to detect the slope of the edges of a character to be displayed. A rise-control signal is generated to control the rise and fall times of the interpolated raster control signal in proportion to the detected slope.